High strength heat resistant alloy steel



May 30, 1961 E. J. DULIS ETAL 2,936,463

HIGH STRENGTH HEAT RESISTANT ALLOY STEEL Filed March 29, 1960 EFFECTS OF ALLOYING ELEMENTS ON CREEP- RUPTURE STRENGTH AT l I F 50 STRESS 2 STRESS 75000 90 000 Si g 200 3 200 p 2: 5 u.I 9 Lu I50 I50 2 o E I: \g I: g I00 A I00 8/g 8 B I; E 0 kg 3 50 5 3 50 O: 0 m 90 CARBON 0%) CHROMIUMP/o) Fig.l Fig.2

, STRESS STRESS a 90000 psi 90 000 psi 3 3 200 E :1: g I g I50 I: I: I I00 g I00 3 I n. g 50 D 50 [Z O 0 I 2.0 6.0 I00 0 0.0I 0.02 0.03

MOLYBDENUMP/o) BORONP/Q) Fig.3 Fig.4

INVENTORS Edw0rdJ.Dulis August KoscIk 8I Vijcy K. Chondhok BY fiw ATTORNEY United States Patent HIGH STRENGTH HEAT RESISTANT ALLOY STEEL Edward J. Dulis, Pittsburgh, August Kasak, Bridgevllle, and Vijay K. Chandhok, Pittsburgh, Pa., assiguors to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey Filed Mar. 29, 1960, Ser. No. 18,415

9 Claims. (Cl. 75-126) This invention pertains to high strength-heatresistant alloys and, more particularly, to heat-treatable martensitic steels having increased strength at ordinary temperatures as well as upon prolonged exposure to high temperatures.

Of the strongest currently available constructional steels for elevated-temperature applications, the heattreatable martensitic elevated-temperature steels, for example, AISI Type H-ll (Vascojet 1000, Crucible 218), Crucible 422, etc., have a combination of outstandingly high strength at ambient or ordinary temperature and at temperatures up to about 1000 F. However, at temperatures above 1000 F. for even short periods of time, these steels lose their strength precipitously. Therefore, for applications at temperatures above about 1100 austenitic steels and superalloys are used rather than martensitic steels. However, the austenitic steels are considerably lower in strength than some martensitic steels in the range from ambient temperature to about 1100 F. Therefore, for applications that require high strength Within the entire temperature range of operation from ambient to the upper limit, a need exists for new martensitic steels that maintain high strength at temperatures above 1000 F. and preferably up to about 1200" F. The permissible higher operating temperatures of highly stressed components would significantly increase the efiiciency of many units that operate at elevated temperatures.

Therefore, it is an object of this invention to provide new alloy steels that are capable of exhibiting very high strength after the conventional hardening and tempering heat-treatment and of retaining unusually high strength at elevated temperatures up to about 1200 F.

It is another object to provide such high-strength, heat resistant martensitic (ferritic) steels having also a sufficient ductility for most constructional applications.

It is a further object of the invention to provide a method for heat-treating the steels of the invention so as to realize and maximize the aforesaid desirable characteristics of our steels.

In accordance with the foregoing objects, there is provided a family of steels comprising, on a weight percent basis, 0.3 to 0.6 percent carbon, 0 to 2.0 percent manganese, 3 to percent chromium, up to 3 percent vanadium, up to 10 percent molybdenum, up to 12 per.- cent cobalt, up to 0.1 percent boron, up to 10 percent tungsten, up to 3 percent columbium, up to 0.5 percent tantalum and up to 2.0 percent titanium, balance substantially all iron. In addition to the proper balance 1 Registered trademark of Vanadium-Alloys Steel Company. Trademark or Crucible Steel Company or America.

2,986,463 Patented May 3 0, 19 61 of carbon, manganese and chromium within this broad range, aggregates of alloying elements may be selected from one of the following groups: (1) 10 to 16% of Y Vanadium, molybdenum and cobalt, with an upper limit of 8% for molybdenum and cobalt, and an upper-limit of 3% for vanadium, plus up to 0.02%boron; (2) 10 to 18% of vanadium, molybdenum, tungsten and cobalt, with an upper limit of 8% for molybdenum, cobalt and tungsten and an upper limit of 3% for vanadium; (3) 10 to 19% of vanadium, tungsten and cobalt, with an upper limit of 10% for tungsten and cobalt and an upper limit of 3% for vanadium; (4) 11 to 18% of vanadium, molybdenum, cobalt, columbium and tantalum, with an upper limit of 8% for molybdenum and cobalt, and an upper limit of 3% for vanadium and for columbium plus tantalum plus up to 0.02% boron, and (5) 12. to 16% of vanadium, molybdenum, cobalt and titanium, with an upper limit of 8% for molybdenum and cobalt, an upper limit of 3% for vanadium, and an upper limit of 2% for titanium, plus up to 0.02% boron. The steels of the invention may also contain'the usual steelmaking impurities, as sulfur, phosphorus, silicon, as well as trampelements, as nickel and copper, in nominal amounts.

These steels have an essentially martensitic structure and combine excellent strength at both ambient and elevated temperatures with other desirable properties, such as a comparatively low coefficient of thermal expansion,

high thermal conductivity, as well as the high damping capacity conferred by the martensitic or basically ferritic structure thereof. Moreover, the ductility of steels of the invention is sufficiently great to fit them fora about 1200 F. For example, the steels of the invention are admirably suited for the construction of airframe and missile structures and jet engine parts subjected to a wide temperature variation. Other novel features and advantages of the present invention will become apparent by reference to the following detailed description when considered in conjunction with the appended drawings, zwherein:

Figures 1-4 are graphical plots illustrating ourfindings of the effect of the alloying elements carbon, chromium, molybdenum and boron, respectively, upon the 1100 F. creep rupture strength of martensitic steels.

Referring now in more detail to the novel steels, the invention comprises the following preferred compositions within the scope of the broad analysis:

Analysis, Welght Percent Element A B O D E GAO/0.45..- 0.30/0.45 03010.45-.. 030/045.-. 0.35/0A0. 0.1/1.0---" 0.1/1.0-.- 0.l/1.0 0.1/1.0"". 7/9 7/9 7/9 7 7/9. 1 0.5/1.0 O.5/1.0 0.5/1.0 0.4/0.6"-.. 0.5/1.0. 5/7.0 5.5/7.0.-- 5.5/7.0 /6 5/7.- 4.5/8 4.5/7 4.5/7 5 6.5/8 5/8. up to002. up to002- up to 0.02- upto0.0l. 1/10 uptol.6. 0.7/1a 0.1/0.5. uptol.0.;-, 0.5/1.5--- uptoLO. balance". balancebalance--. balance.-.

It is necessary that, within each of these compositions, TABLE II the amounts of the austeniteand the ferrite-form ng ele- Roam temperatur tensile properties of experimental merits be so balanced that an essentially martensitic misteels crostructure is obtained upon heat. treatment. More- 5 V 0.27 Oft- Tensile Eionga- Reducover, although the steels of the invention are 1nherentl, Harm set 0,18} d Strength mum mm of capable of being heat treated to extremely high strength 85001100. p ss st e gh 1, 1 111011 $50 I 1 D6!- (31- levels, by reason of the novel compositions thereof, atpsi.) 05111) cont) tainment of the optimum desired properties is also dependent upon the choice of a proper heat treatment. g2 g For example, varying the austenitizing temperature may "5? 328 2 if; 50 200 310 0 11 signlificantly afieet the strength and ductility of the g2 gg G 12 5 0 17 stees 55 242 200 8 20 In the development of our novel steels, a large nurn- 57 204 310 0 i1 250 200 0 10 her of steel compositions were prepared and sub ected 56 274 322 4 10 54 2 to tests for hardness and room temperature and elevated 56 93 g temperature tensile strength and creep-rupture strength Z33 to determine suitability of the steels for the intended 2g g purpose. The effects of different alloying elements, in 2 2g varying amounts, upon these properties were determined. I 0 10 Illustrative of the steel compositions investigated are 54 m 303 4 8 53 241 those appearingin Table I. 51 244 205 2 10 55 254 304 5 20 TABLE I Chemical compositions of experimental steels Weight; Percent 05001100.

0 Mn Gr v M0 00 W Cb T8 T1 B TABLE III In. addition to the elements listed in Table I, the steels Tensile properties of experimental Ste 81s at 11003 F contained 0.014 to 0.019% sulfur, 0.002 to 0.012% I phosphorus, 0.22 to 0.56% silicon, 0.4 to 0.12% nickel; 60 in all cases, the balance was iron. 9 e sile- Elouga- R 0ducset Yield Strength tionin. 1.101101 The steels of Table I were air-induction melted as 30 t Strength (1.000 1 111511 41-50 pound heats, then processed to the form of Aa-inch square and/or A't-ihch square bars. For the purpose of determining the tensile properties of our steels, standard 14 0 J! round A-i-neh tensile test specimens were prepared by 185 1g 27 machining the test composition bar stock'to a standard *3 gauge diameter of 0.250 inch, the specimens having a gauge length of one inch. These specimens, after being 11 31 austenitized at 2100 F. for one-half hour, air cooled, and tempered at 1100 F. for 2+2 hours, were then g 1? sub ected to conventional tensile test procedures, at room 9 temperature and at 1100" F., with results as shown in Tables II and 111, respectively.

I Austenitizedat 2200 F. tor 2+2 hours.

for V; hour, air cooled, tempered at 1100 F The elfect The nominal base composition of the TABLE IV tially constant, in order to show the effect of variation in amount of each of these elements upon the creep-rate and the creep-rupture strength of our steels.

upon these properties of the simultaneous variation of 5 the amounts of a number of these elements is also shown in the table.

steels (obtained by averaging the unintentional vari ations for each element) was 0.38 C, 0.20 Mn, 7.16 Cr,

0.99 V, 5.77 Mo and 4.90 Co. Standard inch round specimens, having a standard gauge length of inch,

were prepared for these tests, in a conventional manner,

from the respective experimental steel compositions and,

after being austenitized at 2100 F. for /2 hour, air

cooled and tempered, for 2 plus 2 hours, at 1100 F.,

were subjected to conventional creep-rate and creep rupture tests at 1100 F.

rupture and creep properties at 1100 F.

Eflects o'f variation of the alloying-elements on the creept 1 88 U mwmwx n m Rw u u {LC r M (h t 75 9 1. e 1mm in em n n on an nm w n human n m 5 2 H dnrm eOA an m e r H 3995 37 98988 58 82 985 7877 848 89812 2 9 4 8 m a \l 1. 1 1 1 11 1 2 .0 D. H H m m u 5555 0 000 00 0o 0 0 00 000 o 0 m m ilfi lt jl l o n n u a u n u n u w n a n u u u n n u u m a u n u n u u u u m. n m 8 D. 1... r o 1 Em I I I H" m m. n u o u M w o u n 2 5 o as "4 2 7 u a 91 a m .2 n An a 0 II I u I I" .I 2 N a 1 1 2 3 4 2 2 7 422 2 7 7 2 6 9 2 nf 2 9 5 2 7 0 W 1111 14 3 31 11 211 133 1 1 1 2 5 5 M CCCC CC 0 CC C0 000 C000 0 C O C C C FFFF FF F FF FF FFF FFFF. F F F F F F AAAA AA A AA AA AAA AAAA A A A A A A i' rom Table II, it will be seen that, at room temperature, the 0.2% ofiset yield strengths of the experimental steels ranged from 208,000 p.s.i. to 277,000 p.s.i. and that the tensile strengths of the steels ranged from 250,- 000 p.s.i. to 322,000 p.s.i. At 1100 F., the steels exhibited, as shown in Table III, a high degree of retention of strength, the 0.2% ofiset yield strengths ranging from 111,000 p.s.i. to 166,000 p.s.i., and the tensile strengths ranging from 168,000 p.s.i. to 201,000 p.s.i.

The steels of the invention also show very low creep 10 rates and possess remarkably high creep-rupture strengths. In Table IV are set forth the results of conventional creep-rate and creep-rupture tests conducted on a number of compositions illustrative of our novel steels and wherein the amounts of the respective elements C, Mn, 15 Cr, V, Co, Mo, C-b+Ta and B were varied, the amounts of the other alloying constituents being held substanassures The P on unce sfi c 9 he el m nt .3 Cr M n B upon e re u u e ren 9f t e h e tsm ats t e i sr h s v ll st t b p n a abscissae, in Figs. 1- 4, respectively, the percentages of these four elements, against the rupture times, in hours as ordinates, for fixed stress loads applied at a tempera ture of 1100" F., as given in Table IV.

In Fig. 1, the graph A clearly illustrates the efiect of carbon in producing in the basic steel compositions an increase in creep rupture time up to a maximum carbon content of about 0.39 percent and a decrease in creep rupture time with further increases in the amount of carbon beyond that value. This effect of carbon upon the contemplated steel compositions is wholly unexpected and, due to this effect, we limit the carbon in our steels to a critical amount of from about 0.30 percent to about 0.45 percent, thereby encompassing the peak of the graph A. In Fig. 1, as well as in Figs. 2-4, the numerals appearing at the measurement points of the respective graphs, indicate the percent elongation of the test specimen at fracture.

Similarly, Fig. 2 illustrates the unexpected benefits conferred by utilization of a critically limited amount of chromium. Graph B of Fig. 2 shows a rise in rupture time to a maximum corresponding to about 8% chromium and a falling off thereafter to a chromium content of about 12 percent whereupon further increases of chromium have little effect upon rupture time, although slightly higher percentages, e.g., up to 15%, may be useful for conferring added corrosion and oxidation resistance to our steels. Accordingly, we place a maximum value of 12-15% upon chromium in our steels, but prefer to hold that element to a maximum of about 9%. Although as little as 3% may be utilized in steels of the type herein contemplated, rupture time, hence strength, is unacceptably low for high temperature applications with a chromium content below about 6 percent and we, there. fore, prefer that percentage as the lower chromium limit for our steels.

In a like manner, graph C of Fig. 3 illustrates our discovery of the criticality of the amount of molybdenum in the steels of the invention, rupture time rising to a maximum at a molybdenum content of about 7% We accordingly prefer a molybdenum content of from about percent to about 7 percent in our steels, thereby incorporating maximum rupture strength encompassed by the peak of graph C.

Steels of the type herein contemplated possess good creep-rupture strength even in the absence of boron.

However, when this element is incorporated in our steels in critical amounts, it results in a maximization of creeprupture strength as illustrated by graph D of Fig, 4 Wherein rupture time at constant stress is seen to rise to a peak at a boron concentration of about 0.009 percent, thereafter decreasing with increasing boron content until, at a boron content of about 0.016, little further change in rupture time is observed with additional increases of boron. Accordingly, we prefer boron in our steels up to about 0.02 percent although boron may be present in greater amounts, e.g., up to about 0.1%, without adversely affecting the creep-rupture strength of our steels. The addition of boron in the preferred range wasfound to reduce the tendency of carbides to coalesce atthe grain boundaries during prolonged exposure of the steels at 1100 F., thereby possibly accounting for the superiorcreep-rupture strengths of our boron-containing steels.

When our boron-containing steels are austenitized at or above a temperature of 2200 F., signs of incipient melting (due, we believe, to formation of an Fe-C-B eutectic) are noted preferentially at the three grain junc-. tions resulting in a tendency for hot crackingduring welding. This tendency, We have found, may be overcome bythe addition of titanium, in amounts of at least about 0.4% or of columbium plus tantalum, in amounts of about 1.0 percent or less to our boron-containing steels.

M reov r. u b onn a ium-s m n s el e ian a m m s ttn o a bide Pre ip tati at stain boundaries. Consequently, in one aspect of our invention, we add to our steels, especially those containing boron, up to about 2%, preferably 0.5 to 1.5% of titanium, up to about 3%, preferably 0.7 to 1.3% of columbiurn, up to about 0.5%, preferably 0.1 to 0.2% of tentalurn, in order to further enhance the excellent combination1 of creep-rupture strength and rupture ductility of our stee s.

Cobalt is a preferred ingredient of our steels, it having been found to be etfegtive in decreasing the rate of softening of the steels upon tempering. However, we find that excessive amounts of cobalt, if utilized together with chromium on the high side of its range, tend to the retention of austenite after the austenitizing step. Accordingly, we limit cobalt to maximum of 12% and prefer a cobalt content between about 4.5% and 8% Vanadium in a maximum amount of about 3% is effective in increasing the hardness and strength of our steels. However, we find that vanadium tends to lower the strength properties of our steels when it is increased substantially over about 1.0%. Hence, we limit vanadium to a maximum of 3% and, preferably, to less than 1%. Especially desirable are steels containing from about 0.5 to about 1.0% vanadium.

We have further found that highest strengths are obtainable with the incorporation of up to 10%, preferably up to about 1.5% tungsten in our steels.

Manganese is included in our steels where sulfur is present in order to eliminate hot-shortness. Manganese is not required where sulfur is absent or substantially so, as in the case of very clean, vacuum melted steels. Accordingly, our steels may contain from 0 to about 1% g manganese.

Creep-rupture evaluations of some of the experimental steels ofthe invention were also made at temperatures of 1000 and 1200 F. For example, when steels AFC21 and AFCZZ were creep-rupture tested at 1000 F. under a stress of 150,000 p.s.i. (after austenitizing at 2100 F. for /2 hour, air cooling, and tempering at 1000 F. for

7 2+2 hours), results were obtained as shown in Table V:

These results show that the -hour creep-rupture strength of the steels of this invention at 1000" F. is significantly greater than 150,000 p.s.i. To the best of our knowledge, the creep-rupture strengths of the steels of this invention are greater than that of any other known steel at 1000 F.

Results obtained in creep-rupture tests at 1200 F. showed that the steels of the invention also retain useful properties at that higher temperature. For example, steel AFClS, when creep-rupture tested at 1200 F. under a stress of 35,000 psi. (after austenitizing at 2200 F. for /2 hour, air cooling, and tempering at 1100 F. for 2 hours), had a rupture time of 112 hours; the rupture elongation and reduction of area were 36 and 68%, respectively.

Comparison of the tensile strengths, at both room temerature and at elevated temperatures, e.g. 1100 F., of the steels of this invention with those of prior art high strength, heat-resistant stainless and hot-work steels of both the austenitic and the martensitic types shows the inherent superiority of our steels. The results of such comparisons are set out in Table VI.

10 TABLE VI comparison of tlie tertsil strengths of steels of this invention and those of typical prior art austenitic and marterisitic' stainless arid hot-work steels Test Tensile A I St'eelType Designation Condition Temp. Strength Source F.) (1,000 p.s.i.)

Ansteni'tic Stainless AISI 316; Annealed and Cold Room 90 "Stainless and Heat Resisting 1 Drawm Steels," AISI, June, 1957. Martensitic Stainless Crucible 422 -1 Quencgen; 33161 1g lenido 260 Product Engineering, 1957.

pere a Hot-Work AISI Type H-11 Quenched and Temdo 260 ASM, High Strength Steels for (Vascoiet 1000, pered at 1,060 F. Aircraft, May 1958. Ctrincible 218, e c. t This Invention Quenched and 'Ie'rndo 250 to 322 pered at 1,100 F. Austenitic Stainless 18 Cr-8 Ni+M0. Anneale .1 1,100 67 ASTM STPND. 124. 1 V Martensitic Stainless Crucible 422 Quenched and Tem- 1,100-... 88 Unpublished Data oi Crucible pered at 980.11. Steel Co. of America. Hot-Work AISI Type H-11 Quenchedandflem- 1,100 151 ASM, High Strength Steels tor pered at 1,050 F. Aircraft, May 1958. This Invention quenched and Tern- 1,100---. 178 to 201 pered at 1,100 F.

Inspection of Table VI reveals that, at room temperature, the steels of the invention possess tensile strength as high as or higher than those prior art steels as AISI H-ll or Crucible 422 commonly accepted as among the best currently available high strength, heat resistant steels for high temperature constructional applications. More importantly, however, Table VI clearly shows the superiority of our steels in respect to their ability to retain high tensile strengths at elevated temperatures, e.g. l100 F. Whereas,- the tensile strength of Crucible 4'22 niartensitic steel drops from avalue of 260,000 p.s.i. at room temperature to a value of 88,000 p.s.i. at 1100 F., and that of AISI Type H-ll steel drops from a value or 260,000 p.s.i. at room temperature to a value of 151,000 p.s.i. at 1100 F., the tensile strength of the steels of this invention, exhibiting tensile strengths of from 250,000 to 322,000 p.s.i. at room temperature, retain excellent tensile strengths of from 168,000 to 201,000 p.s.i. at 1100 F. This characteristic, as aforesaid, admirably suits our steels for high temperature constructional purposes.

In Table VII there are shown the results of comparisons of the creep-rupture strengths at temperatures of 1000" F., 1100i F. and 1200 F., of the steels of this invention with those of typical austenitic and martensitic high temperature steels of the prior art.

TABLE VII We have found that, in general, asuitable heat treatment for our novel steels consists of (1) austenitizing a't' 2100 F., 2) air cooling and (3) tempering, for 2+2- hours, at 1100 F. Such treatment gives products showing an excellent combination of strength and ductility in citeep-rupture' tests at 1100 F. Higher austenitizing temperatures slightly increase the creep-rupture strength of our steels, but tend to reduce the ductility thereof. However, it has been found that additional improvement of hot strength may be obtained by the use of an isothermal treatment comprising transferring the steel from the austenitizing step to a constant temperature bath and holding at a certain constant temperature for a certain period of time, followed by air cooling to' room tempera: ture. Thus, isothermal treatments, comprising holding the previously austenitized steel (1) at 400 F. for eight hours or (2) 700 F. for four hours were found to Inaterially increase the creep-rupture strength and ductility of our steels. For example, when creep-rupture tested at 1100 F. under an applied stress of 75,000 p.s.i., after austenitizing at 2200 F. for one-half hour, air cooling to room temperature and tempering at 1100 F. for 2+2 hours, steel AFClG (Table I) had a rupture life of 236 hours and an elongation of 1%. After the same austeni tiz'ing treatment, followed by isothermal treatment at at 1000, 1100, and I200 F.

700 F, for four hours, the same steel had a rupture life of 3" 68l1ours and the rupture elongation was 9%.

Similarly, steel AFCZl (Table I), after similar iso thermal treatment, at the lower temperature of 400 F.,

IOU-Hour Creep lliupture, Strength (1,000 p.s.i.) at Steel Type Designation Source 1',000 F. 1,100 F. 1 ,206 F.

is cr-sNL .1 4s 34 2a ss'r s'ih No. 124. IBCt-B Ni-l- 45 a2 ASTM sTP No.124. .1 AISI Type H-11 87 Iron Age: fSurvey 0t Hot-Work Tool Steels for Aircraft and it Missiles, February 12, 1958. 2% 25. ASTM STP No. 170-A. Egg 26 ASTM- STP N0. 170*A. 97 11:11:11 35 It will be seen that, in the case of each of the three temperatures at which creep-rupture strength was studied, the, steels of the invention possess higher creep-rupture strengths than any of the prior art steels with which compersons were made. 75

upon 1100 F. creep -rupture testing, showed a 100 hour 11 creep-rupture strength of 115,000 p.s.i. After the same heat treatment, and upon creep-rupture testing at 1000" F., steel AFC22 (Table I) showed a 100 hour creeprupture strength greater than 175,000 p.s.i. 1

Still further increases in strength, together with a high ductility, may be achieved by tempering steels of the invention immediately after a short, e.g., 15 minutes, isothermal treatment without an intermediate air cool. This is shown in Table VIII by the results of creep-rupture tests performed upon steel AFCl7 (Table I) after specimens of the steel had been subjected to difier ing heat treatments.

TABLE VIII strength of at least about 210,000 p.s.i. and a tensile strength of at least about 280,000 p.s.i., and capable of retaining an 0.2% offset yield strength of at least about 110,000 p.s.i. and a tensile strength of at least about 170,000 p.s.i. upon prolonged exposure at a temperature Effect of heat treatment upon creep-rupture and creep strength Rupture Minimum Test Stress Creep Steel N 0. Heat Treatment Temp. (1,000 Time Elong. Bad. Rate (Per- F.) p.s.i.) (111:) (Per- Area cent/ cent) (Perhr.) X10 cent) 2,100" F., hr., AC. 153 9 2.1 ,100 F., 2+2 hr AFC" F. 1,100 76 239 1 0.4 AFC" 1,100 75 309 3 4 0. 87 AFG17 1,100 75 509 7 6 0.25

10 min., 1L0

I Air-cool.

The last-mentioned heat treatment has been observed to produce steels having hard martensitic grain interiors and ductile tempered-bainitic (or martensitic) grain boundaries. Upon application of stress, the grain boundaries deform in a ductile manner and hence are capable of accommodating severe stress concentrations without crack formation. Accordingly, the microstructure of our novel steels, as a result of the aforesaid heat treatment, possesses an enhanced resistance to embrittlement and, hence, to premature rupture at grain boundaries. 1 It will be seen, therefore, that we have provided a group of steels possessing to a high degree the desirable combination of great strength and creep-resistance at both room and elevated temperatures, together with sufiicient ductility for most constructional applications. It will be understood that the foregoing description and specific examples are illustrative of the broad principle of our invention and that additions and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention.

We claim as our invention:

l. A heat-treatable alloy steel comprising 0.30 to 0.45 carbon, 0 to 1.0% manganese, 6 to 9% chromium, 0.5 to 1.0% vanadium, 5 to 7% molybdenum, 4.5 to 8% cobalt, up to 0.02% boron, and the balance substantially all iron, said steel being characterized by very high tensile and creep-rupture strengths at temperatures up to 1200 F.

2. A heat-treatable alloy steel comprising 0.30 to 0.45 carbon, 0.10 to 1.0% manganese, 7 to 9% chromium, 0.5 to 1.0% vanadium, 5.5 to 7.0% molybdenum, 4.5 to 7.5% cobalt, up to 0.02% boron, balance substantially all iron, said steel being characterized by very high tensile and creep-rupture strengths at temperatures up to 1200 F.

3. A high strength heat resistant steel consisting essentially of 0.30 to 0.60% carbon, 3 to 15% chromium, 0.5 to 3.0% vanadium, 5 to 10% molybdenum, 4.5 to 8% cobalt, up to 0.02% boron, up to 10% tungsten, up to 2% titanium, up to 3% columbium plus tantalum, balance substantially all iron, said steel being characterized in having, atroom temperature, an 0.2% offset yield 5. A heat-treatable alloy steel comprising 0.30 to 0.45% carbon, 0.1 to 1.0% manganese, 7 to 9% chromium, 0.5 to 1.0% vanadium, 5 to 10% tungsten, 4.5 to 7.5% cobalt, and balance substantially all iron, said steel being characterized by very high tensile and creep-rupture strength at temperatures up to 1200 F.

6. A heat-treatable alloy steel comprising 0.30 to 0.45 carbon, 0.1 to 1.0% manganese, 7 to 9% chromium, 0.5 to 1.0% vanadium, 5.5 to 7.0% molybdenum, 4.5 to 7.5% cobalt, 0.7 to 1.3% columbiurn, 0.1 to 0.5% tantalum, up to 0.02% boron, and balance substantially all iron, said steel being characterized by very high tensile and creep-rupture strength at temperatures up to 1200 F. 7. A heat-treatable alloy steel comprising 0.30 to 0.45 carbon, 0.1 to 1.0% manganese, 6 to 7% chromium, 0.4 to 0.6% vanadium, 5.0 to 6.0% molybdenum, 6.5 to 8.0% cobalt, 0.5 to 1.5% titanium, up to 0.02% boron, and balance substantially all iron, said steel being characterized by very high tensile and creep-rupture strength at temperatures up to 1200 F.

8. An alloy steel comprising 0.30 to 0.45% carbon, 0.10 to 1.0% manganese, 6 to 9% chromium, 0.5 to 1.0% vanadium, 5 to 7% molybdenum, 4.5 to 8% cobalt, up to 0.02% boron, up to 10% tungsten, up to 3% columbium, up to 0.5% tantalum, up to 1.5% titanium, balance substantially all iron, said steel being characterized in having very high tensile and creep-rupture strengths when austenitized at about 2100 F. for one-half hour, air-cooled and tempered at 1100 F. for 2+2 hours.

9. A high strength, heat-resistant steel comprising 0.35

' to 0.40% carbon, 7 to 9% chromium, 0.5 to 1.0%

pup to 1.5% tungsten, up to 0.01% boron, and balance 0 substantially all iron.

References Cited in the file of this patent UNITED STATES PATENTS De Vries Aug. 20, 1940 'Geotfrey et a1. Aug. 19 1958 

1. A HEAT-TREATABLE ALLOY STEEL COMPRISING 0.30 TO 0.45% CARBON, 0 TO 1.0% MANGANESE, 6 TO 9% CHROMIUM, 0.5 TO 1.0% VANADIUM, 5 TO 7% MOLYBDENUM, 4.5 TO 8% COBALT, UP TO 0.02% BORON, AND THE BALANCE SUBSTANTIALLY ALL IRON, SAID STEEL BEING CHARACTERIZED BY VERY HIGH TENSILE AND CREEP-REPTURE STRENGTHS AT TEMPERATURE UP TO 1200*F. 